This invention relates to an apparatus that encodes video signals efficiently with fewer codes and an apparatus that decodes the coded signals, in their use for recording, transmission and display systems, and more particularly to a coding/decoding apparatus for video signals possible of random access and search.
In particular, efficient coding has been realized by interframe predictive coding which utilizes interframe correlation of video signals to predict, from already coded frames, the frames to be coded, and encodes predictive error signals alone.
Furthermore, recent attention has been directed to technology of motion-compensating interframe prediction to predict motion by moving pictures.
On the other hand, it has been made for the coding for storage media to respond to random access and high-speed search.
A conventional coding apparatus is exemplified in FIG. 1.
In FIG. 1, a video signal applied to a video signal input terminal 1 is led to a predictive subtracter 3, where a predictive signal given via a switch 15 is subtracted from the video signal, thus generating a predictive error signal.
The predictive error signal is encoded by an intraframe encoder 5 to be compressed data. The data is outputted via a data output terminal 7 and applied to an intraframe decoder 9 which generates a reproduced predictive error signal.
In an inverse predictive adder 11, a predictive signal is added to the reproduced predictive error signal, thus reproducing the video signal.
The reproduced video signal is stored in a frame memory (FM) 13, and applied to a terminal P of the switch 15 after delay by one frame.
The switch 15 is controlled by a synchronous signal separated from the inputted video signal. And, in an independent frame of every N frames, a terminal S is connected to a terminal I, whereas in other predictive frames the terminal S is connected to terminal P.
The terminal P has been supplied with a reproduced video signal of the preceding frame. An interframe predictive coding circuit is formed when the terminal S is connected to the terminal P.
The terminal I has been supplied with a fixed value (0). When the terminal S is connected to the terminal I, the output signal of the predictive subtracter 3 becomes the same as the input video signal, that is, an intraframe independent coding circuit is formed.
FIG. 2 shows a conventional video signal decoding apparatus. In this apparatus, the data supplied from a data input terminal 17 is decoded by an intraframe decoder 19.
And in a reverse predictive adder 21, a predictive signal applied via a switch 27 is added to the decoded signal, thus reproducing the video signal.
The reproduced video signal is outputted via a video signal output terminal 23, and is concurrently stored in a frame memory 25.
The signal is delayed by one frame in the frame memory 25, and then applied to the reverse predictive adder 21 via the switch 27. The switch 27 is controlled by a frame-synchronizing signal separated from the input data as in the case of the coding apparatus.
In the intraframe coding apparatus 5 of FIG. 1, discrete cosine transform (DCT) is first carried out. The transformed output is then quantized, and the quantized data is variable-length encoded by such codes as Haffman codes.
In the intraframe decoders 9 and 19 of FIGS. 1 and 2, the data coded to variable lengths is first decoded to a fixed length and then transformed to a quantized representative value, which is reversely discrete cosine transformed to a reproduced signal.
Such conventional coding/decoding apparatuses process periodical independent frames. This decreases the coding efficiency because the independent frames produce a large amount of data as compared with the predictive frames.
Japanese Patent Application NO. 3(1991)-125393 by the same applicant as the present invention discloses video signal coding/decoding apparatuses.
In the coding apparatus, a video signal of the present frame is coded and is then decoded. The decoded signal including the video signal and quantization error is subtracted from the video signal to generate a difference signal.
The difference signal is added to the video signal in coding of a video signal of the next frame. This results in coded data including the video signal of the next frame and the quantization error of the preceding frame with the opposite polarity.
In the decoding apparatus, decoded video signals of the present and the next frames are added to each other (inter frame addition) to cancel the quantization error generated in the coding.
As a result, a reproduced video signal keeps better image quality even if the number of quantization steps in the coding is made less. An amount of the coded data thus can be made less.
When a pass band of a time direction low pass filter (LPF) realized by the interframe addition is narrowed, that is, almost the coded data of the preceding frame is used in time integral in the interframe, image quality is improved if there is less image change in the interframe.
On the other hand, image quality will be deteriorated if there is a motion picture. Therefore, the pass band of the time direction LPF cannot be narrowed so much and the amount of the coded data cannot be made less in motion picture signal coding.